Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
887 FOUS30 KWBC 150044 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 844 PM EDT Sat Sep 14 2024 Day 1 Valid 01Z Sun Sep 15 2024 - 12Z Sun Sep 15 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEASTERN U.S... ...Southeast... ...0100 UTC Update... Based on the latest observational trends (including mosaic radar and IR/WV satellite loops and SPC mesoanalysis), along with the recent HRRRs and 18Z HREF suite, have pared both the Slight and Marginal Risk areas at 0100Z. Elongated NW-SE oriented mid level vort lobe across the Mid-South (incorporating remnants of Francine) continues to undergo shear between the 588dm H5 ridge centered across the eastern Great Lakes and the flat 588dm ridge nudging into the Southern Plains. Along this ribbon of better upper level forcing between the two large-scale ridges is an accompanying uptick in low-level moisture transport along an axis of higher TWP (1.75-2.00"). However, convective trends have been on the downswing of late this evening given the diminishing deep-layer instability (negative MUCAPE trends over the past 3 hours per the SPC mesoanalysis). This has resulted in some decay in both coverage and intensity of the convection, however the deep-layer forcing and thermodynamic profile will nevertheless support 1-1.5"/hr rates underneath the strongest cores overnight. Have confined the Slight Risk over areas in AL and southwest GA where the recent rainfall was heaviest (current soil moisture percentiles aoa 95%), and as such where 3 hourly FFG values are as low as 1-1.5" in spots. Hurley ...Previous Discussion... Radar and satellite imagery continued to show a broad cyclonic gyre situated across the southeastern U.S with a north- south band of convection bisecting the a good portion Alabama into Western Tennessee as of mid-morning. The surface reflection from what had been Francine will continue to become more diffuse over the next 6 hrs with a degrading surface signature, leaving much of the flow tied to the leftover mid and upper lows associated with the storm. That said, the cyclonic spin from the tandem 850-500mb low(s) will allow for a continued favorable moisture stream around the circulation with small mid- level perturbations stuck over the same areas allowing for locally enhanced ascent within the confines of Alabama, Georgia and Tennessee. Precipitable water values generally residing within the +1 standard deviation anomaly across the Deep South will be sufficient enough within the pattern to yield another round of scattered to widespread coverage of showers and thunderstorms across similar areas to those areas that were impacted on Friday. The difference is the magnitude of the convective pattern will be a bit less vigorous thanks to the weakening mid and upper low centers as denoted within all the recent guidance height fields. Still, the combination of the moisture anomaly and semi- favorable ascent under the remnant mid and upper lows will be plenty to offer another round of convection across the Deep South. To the northeast, a stout surface ridge will nose down further into the interior Southeast with a formidable easterly regime focused in-of Northern GA down into the Northwest side of AL. The combination of the cyclonic flow from the mid-level remnants and wedging associated with the surface ridge will create a tightened axis of convergence within the state of Alabama down into western Georgia. The 14/12Z CAMs available through 15Z remained well represented within the HREF mean QPF and associated neighborhood probability field. Given the expanding footprint of diminished flash flood guidance, the prospects of excessive rainfall will be increasing with additional rainfall. Signals for 1-2"/hr rainfall rates would also be sufficient to result in run off and flooding given the antecedent conditions. A higher end SLGT risk still seemed reasonable for this the forecast for a northwest to southeast oriented axis from northwestern Alabama to far southwest Georgia. ...Coastal Carolinas... Satellite and radar imagery showed showers and thunderstorms hugging the immediate coastline of near the Northern Carolina and South Carolina border with the heaviest rainfall off-shore. The convection was largely being driven/focused by a weak mid-level circulation that had little reflection at the surface. CAPE values over land are forecast to be pretty modest...on the order of 500 to 1000 J per kg...with broad cloud cover but think there could be enough of an on- shore component north of the circulation center to draw some cells with 1 to 2 inch per hour rates inland. Kleebauer/Bann Day 2 Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST U.S... 2030Z Day 2 Excessive Rainfall Discussion Update... Overall...the previous outlook looked to be standing up well with time and the changes were few and minor. Models still point to deep moisture lingering over the southeast but the overall threat of excessive rainfall should be diminishing as the strength of the upper forcing wanes. Increasing moisture and mid- and upper-level height falls should combine to increase the excessive rainfall concern in the Southwest U.S. as much due to the chance for rainfall rates exceeding 0.3 inches per hour over the most sensitive burn scars and isolated 1 inch rainfall amounts elsewhere. Bann ...Previous Discussion... ...Southeast... Despite the remnant mid and upper circulation from Francine degrading in its presentation, another period of enhanced convective activity will spawn underneath the primary upper forcing lingering over the Southeastern U.S. There`s a bit more emphasis on an axis of convection developing across portions of the Lower Mississippi Valley in-of AR and MS, extending southeast into parts of the Deep South, mainly Southern MS and AL. Areal PWATs will remain elevated within +1 standard deviations, enough of an anomaly to spur up some potential heavier convective cores that could result in some flash flood prospects, especially over any urban zones where run off potential is greatest. Ensemble bias corrected QPF and the 75th percentile of the NBM indicate a fairly stout axis of heavier precip likely to occur under the influence of the upper low and remnant energy pivoting around the general circulation. The primary concerns will continue to be those locally heavy rates that will reach between 1-2"/hr with some intra-hour potential for heavier rates that typically cause more issues in this part of the country. A SLGT risk was added to the previous forecast with an adjustment southwest to the overall SLGT/MRGL risk areas to align with the current trends and ensemble forecast consensus on the heaviest precip location. The MRGL expands into the FL Panhandle to cover the area that has been hit pretty hard in the past 36 hours as isolated storms are still forecast for those areas that have seen copious amounts of rainfall in the above time span. ...Southwest... Remnant moisture from now Tropical Storm Ileana will continue to advect northward into the rest of the Desert Southwest with a trajectory towards the Four Corners, wrapping back eastward into NM during the afternoon hours Sunday. The ensemble QPF footprint upped the total precip forecast compared to the previous periods, a trend that was noted when assessing the QPF differential from 12z to 00z. This places the convective pattern towards those areas we`ve been highlighting all summer for flash flood concerns thanks to a plethora of burn scars located within the terrain across parts of NM. The previous forecast was relatively unchanged over AZ into the Four Corners, but the MRGL risk was expanded to include much of NM with a focus into the terrain over the Sangre de Cristos and the Sacramento Mountains. General QPF average between 0.25-0.75" is expected, but maxima of >1" will be plausible anywhere within the confines of the forecast risk area. A targeted SLGT risk is still possible, but the scattered nature of the convection forecast doesn`t instill enough confidence at this time. A reassessment with the addition of CAMs guidance could sway that the other way, so stay tuned for future updates. Kleebauer Day 3 Valid 12Z Mon Sep 16 2024 - 12Z Tue Sep 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN NORTH CAROLINA... ...2030Z Excessive Rainfall Discussion Update... The 14/12Z model guidance continued to show somewhat better agreement amongst the individual model runs in terms of track and timing of a low pressure system that approaches the southern Mid Atlantic coastline on Monday but the spread was still considerable. The deterministic QPF from WPC tended to be closest to the 14/12Z UKMET run as a compromise between the faster GFS and slower ECMWF. Given the probabilistic nature of the EROs...expanded the Marginal and Slight areas a bit to account for uncertainty in track and in timing rather than an increased confidence in the extent of areal coverage. With that caveat...it does appear that some locally heavy rainfall is possible somewhere to the Southern Mid Atlantic region...generally confined near the coast...beginning late Monday night or in the early morning hours of Tuesday that persists beyond the end of the short- range excessive rainfall outlook period. In the Western United States...the amplification of a synoptic scale trough and lingering moisture should result in an additional round of showers and thunderstorms with rainfall rates high enough to produce isolated run off problems. Saw no reason to make any changes to the on-going Marginal risk area. Bann ...Previous Excessive Rainfall Discussion... ...Coastal Southern Mid Atlantic... A growing consensus for developing low pressure along the Carolina coast has popped up amongst the 00z deterministic with much of the global suite indicating surface cyclogenesis off the GA/SC coast, moving northwest towards the Eastern Carolinas with emphasis on North Carolina based on the latest runs. This is a fairly complex evolution but rooted within the proposed synoptic evolution as a strong block residing over the Northeast and Northwest Atlantic and steering pattern focused towards the Eastern Carolina coast sometime on Monday. The setup would allow for a steady easterly flow to preside over Eastern NC with the approach of the surface cyclone increasing a deep moist advective pattern in-of the coastal portions of the state. The National Hurricane Center (NHC) has marked this potential disturbance with a 50% chance of developing into a tropical system within the next 72-96 hours which would invoke a growing confidence in something more organized that could promote greater tropical characteristics that would undoubtedly allow for some locally greater impacts than forecast. As of now, the SLGT from previous forecast was maintained, but adjusted to the north to account for the ensemble shift in the low pressure track and within the core of the heaviest QPF. The area from Myrtle Beach up through Cape Hatteras will have to monitor the potential closely as the chance for locally significant rainfall amounts have been scaled up since the last update. For now, the SLGT risk will suffice with the opportunity for a higher risk, pending the expected evolution of the disturbance. ...Central Gulf Coast... A weak surface low is forecast to develop in-of the Central Gulf coast by Monday morning with an increasing axis of convergence within the northern flank of the circulation. Elevated PWATs along with prominent instability located along the Gulf coast will amplify the threat of some locally enhanced rainfall prospects between Southern MS through Southern AL into the Western FL Panhandle. The main threat will continue to be urban flooding, but will have to monitor the area around Apalachicola closely thanks to significant rainfall the past few days with more anticipated prior to the period. This has allowed for a sharp degradation of the FFG indices normally in place for the area. A MRGL risk was introduced given the sneaky threat with a lot contingent on the expected development of a weak surface reflection along a quasi-stationary front situated along the Central Gulf coast. ...Western U.S... Increasing moisture nosing north ahead of an approaching deepening long wave trough axis will allow for the opportunity for widespread convective development across much of the Southwest U.S, extending north through the Great Basin and the Interior Pacific Northwest. Signals have been growing steadily for a large scale ascent pattern that would favor locally heavy precip in-of the areas above, especially across NV/UT/ID thanks to a premium diffluent upper pattern approaching the West Coast in the form of a potent trough, turned closed low by Monday morning. The setup would be conducive for totals exceeding 1" over a large area that normally does not see these types of totals very often. The PWAT anomaly forecast within the ensembles are approaching 2+ standard deviations above normal with the latest NAEFS indicating upwards of +3 deviations located across parts of NV into ID by the end of the period. If this setup maintains continuity, there`s an expectation that a risk upgrade may be in order, especially since the upper pattern is favorable for widespread convective impacts into areas with climatologically low FFG indices. Kleebauer Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt